Automated license plate reader (ALPR) systems are typically used for detection and recognition of license plates. One example of an ALPR system uses an infrared camera and an infrared light source for irradiating a license plate with infrared rays. Typically, ALPR systems use an electrical system to detect and check passenger vehicles. Detection and recognition of license plates is useful in, for example, the enforcement of traffic laws, investigation of vehicles used to commit crimes, and for vehicle access control to facilities. Police are also sometimes equipped with mobile ALPR systems in patrol cars.
As mentioned above, license plates may be counterfeited and vehicles having such counterfeited license plates used to commit crimes. Consequently, recognition of counterfeited license plates is desired. Some techniques used to prevent counterfeiting of license plates are mechanical mechanisms such as, for example, embossing numbers and characters on the license plate, using aluminum seals, and the like. However, these mechanical mechanisms cannot be detected by ALPR systems.
Exemplary anti-counterfeiting mechanisms capable of being detecting by infrared rays have been proposed, such as, for example, in Japanese Unexamined Patent Application Publication No. H1-152487 (Patent Reference 1) which describes a hidden printing label, wherein a black organic dye that transmits infrared rays is used to color one or both of a film and preferably an adhesive agent layer provided on one side of the film, an information printing layer is provided in the adhesive agent layer through an ink containing an inorganic pigment that absorbs infrared rays, and a release paper is provided on this printing layer. Japanese Unexamined Patent Application Publication No. H3-173691 (Patent Reference 2) describes a heat sensitive transfer material including a thermal transfer ink layer that contains a compound that selectively absorbs wavelengths from 400 nm to 700 nm, and a thermal transfer ink layer that contains a compound that selectively absorbs light having a wavelength from 700 nm to 1500 nm, provided on a substrate.
Japanese Unexamined Patent Application Publication No. H6-186908 (Patent Reference 3) describes a seal characterized in that a verification layer composed of an infrared absorbing ink and a pattern layer composed of an infrared transmitting ink are provided on the surface of a fragile base sheet, and a release sheet is adhered to the back via an adhesive layer.
Japanese Utility Model Publication No. 02584911 (Patent Reference 4) describes an anti-counterfeiting label characterized by the anti-counterfeiting label including: a label base that has first and second surfaces, an adhesive layer that is formed on the first surface, an infrared non reflective pattern that is formed on the second surface, a shielding layer that is formed on the infrared non reflective pattern, and an infrared transmitting ink material based camouflage layer that is formed on the shielding layer; and the infrared non reflective pattern including an infrared reflective layer formed on the second surface, and an infrared absorbing layer formed in a predetermined pattern on the infrared reflective layer.
Japanese Unexamined Patent Application Publication No. 2007-171956 (Patent Reference 5) describes a low visibility retroreflective visual tag such that, to form the low visibility retroreflective visual tag, the production thereof includes a step that places an infrared shielding material on the retroreflective substrate, the infrared shielding material is placed so as to form a pattern, and when the low visibility retroreflective visual tag is irradiated by an infrared light source, the pattern can be recognized by using an infrared sensor.
Japanese Unexamined Patent Application Publication No. 2007-021998 (Patent Reference 6) describes an infrared absorbing film characterized in that: the infrared absorbing film has a substrate film and a cured product layer having an energy cured resin composition that contains an inorganic pigment provided on one side thereof; the light transmittance over the entire wavelength range from 850 to 1300 nm is 10% or less; and the visible light transmittance is at least 65%.
Japanese Unexamined Patent Application Publication No. H2-145381 (Patent Reference 7) describes an information containing composite layer characterized in that two layers are provided for which visual recognition is impossible or difficult, and these layers possess at least one location having a specific wavelength range within which the spectral reflectivity difference is at least 30% in the near infrared region from 700 to 2500 nm.
A license plate that uses a retroreflective sheet including an infrared non-transmissive material, a reflective layer, and cube corner elements is described in PCT Publication No. WO 2011/046957.